A sweep frequency generator provides an output signal which is maintained at a basic or starting frequency and periodically sweeps linearly through a given frequency range. High linearity sweep frequency generators are used in spectrum analyzers, detection and surveillance receivers, and similar type devices for electrically sweeping through a desired range of frequencies. In general, the accuracy of the apparatus is dependent upon the linearity of the sweep from the generator.
In the prior art, many schemes have been devised for including a variable frequency oscillator in a feedback loop to correct the linearity. An example of one such scheme is described in U.S. Pat. No. 3,382,460, entitled "Linearly Swept Frequency Generator", patented May 7, 1968. In the patent described, the output of a voltage controlled oscillator is sampled at intervals, generally at an integral number of cycles, and, if the generator output frequency changes at the correct rate the sampled phases will all be identical. Any variation in the sampled phase is applied to the oscillator as an error signal. This sampling of the output frequency with time leaves room for error and can be a relatively complicated timing problem. Further, because of the periodic sampling at integral numbers of cycles and the inherent timing problems, this system becomes difficult at high speeds.